What Killed This Bearing?

[smiller]

Out of curiosity I decided to disassemble a front wheel bearing removed from service because it felt a little 'crunchy'. The balls and retainer seemed fine but an inspection of the inner and outer races showed this pitting pattern.

 

I wouldn't exactly call this a premature failure since this bearing had 75k miles on it but I'm still wondering what caused it to die. Since it worked fine for quite some time it would seem that simple age had something to do with it, but there appeared to still be plenty of grease available so it doesn't seem like a lubrication failure. Why did it give up the ghost?

[Hedge36]

Tolerances. They increase exponentially as the wear increases, and all grease is doing is slowing down that wear.

[smiller]

One response? Gee, I thought you guys would be all over this one...

 

Could you expand on what you mean by 'tolerances'? The balls and races seemed to have zero play in all dimensions so I couldn't see an excessive clearance problem causing this.

[matanuska]

One response? Gee, I thought you guys would be all over this one...

 

Could you expand on what you mean by 'tolerances'? The balls and races seemed to have zero play in all dimensions so I couldn't see an excessive clearance problem causing this.

Sorry, didn't notice you had posted a picture the first time.

 

Looks like some type of pitting corrosion to me. Drive through any heavy rain or standing water recently?

 

DR

[ShovelStrokeEd]

Seth,

The inner race is too far gone for me to tell anything but, the outer race is showing a symetric pattern of lines, Might be from some ambient light but I don't think so. That, to me, looks like impact damage. Did you possibly bottom the fork or hit a really sharp edged bump or hole?

 

I remember you saying that the outer race of the free bearing was a pretty tight fit in the wheel. I really don't think that is as it should be. I would expect that bearing to be what is called a line on line fit in the wheel or at +0.001 or 2" clearance. That bearing needs to slide into the wheel and up against the spacer being driven by the axle against the inner race and stopped by the inner race against the spacer. Perhaps someone performming a tire change banged the axle into place and, with the tight fit in the wheel, that could cause the balls to damage the outer race in a pattern similar to what the photo shows. Even a soft, dead blow hammer could cause this kind of damage.

[smiller]

Derek - Yes, it does have the appearance of corrosion but I don't think that's it. The bearing was fully sealed and the seals were intact and the bearing full of grease, and the bike is used on a virtually daily basis. I can't see how any corrosion of that magnitude could have gotten started.

 

Ed - I was about to tell you that those lines were a lens artifact but upon close inspection I see that they are actually there and the photo just makes them more visible. It does look like the bearing has taken a pounding but I don't see how as the bike never goes off-road and I don't remember hitting any particularly sharp bumps at any time in the recent past. The bike just sees typical road use.

 

As for the fit of the bearing, it was tight but exactly the same as the bearing on the other side. And I don't think it was likely damaged by improper maintenance. The only thing that was ever used to start the axle shaft was maybe a tap from the handle of a screwdriver (and I know that for sure since no one else has ever touched a wrench to this bike since it was new.) The bearing is very stout and it's hard to believe that light a tap could harm it, and besides... in order to get the axle started in the wheel it has to be already located in the right-side bearing before one could even begin to tap it home the rest of the way... so no real chance that I could see of the bearing being injured in that manner.

 

I should note one thing that isn't visible in the picture. Under magnification, in a few spots along the race you can see some slight pitting that doesn't look so much like corrosion as it does the early stages of some sort of metal fatigue. I'm not sure what it means but it does look like it could be the early stages of the more severely affected areas visible in the photos.

[bmwmick]

Seth,

Looks like someone took a divot out of that outer race.

The inner race sure looks like a corrosion problem. The marks or 'lines' in the outer race do look like that bearing took a heavy jolt at one time. Are the lines all the way around the outer race? If so, it could have been damaged at the factory and just took this long to actually fail. Were any balls "crunchy" looking?

I put new front bearings in my K11 at 50K just because I was going to be putting some big miles on it. It was a waste of time and money because they came out like new (after disassembly). My RT still has the originals at 46K. The K11 feels great at 138K.

Mick

[smiller]

I'll agree that it sure looks like corrosion, but again I just can't see it considering the operating conditions. Also, as mentioned there are other parts of the race that definitely show some sign of impending metal fatigue. If I didn't know better I'd probably guess that the bearing was overloaded, but I doubt that's the case here as front wheel bearings in this application usually show a very long life. Very mysterious.

 

Are the lines all the way around the outer race? If so, it could have been damaged at the factory and just took this long to actually fail. Were any balls "crunchy" looking?

The lines do exist all the way around the circumference of the race but are at irregular intervals. The balls themselves all appeared to be in perfect condition.

 

 

I put new front bearings in my K11 at 50K just because I was going to be putting some big miles on it. It was a waste of time and money because they came out like new (after disassembly). My RT still has the originals at 46K. The K11 feels great at 138K.

The bearing that failed was the right-side, single-row bearing. The left-side double-row bearing was still in perfect condition as far as I could tell, or at least when rotated it felt perfectly smooth and just as good as the new replacement. I don't normally replace wheel bearings as PM but rather check them at every tire change and have spares on hand just in case. This is the first time I've ever needed one, hence my curiosity.

 

[bmwmick]

I don't normally replace wheel bearings as PM but rather check them at every tire change and have spares on hand just in case. This is the first time I've ever needed one, hence my curiosity.

 

It was a good 'catch' any way you look at it. Just another reminder to ALWAYS check the bearings when replacing rubber.

 

Mick

[Chrish1234]

Are the races chromed and is the chrome coating failing due to wear and fatigue?

 

Chris

[Halk]

Electric current causes wierd things to happen to metals, especially 'dissimilar' metals. The building I work in has an electrical problem in my end of the building. The copper water pipes are constantly springing little pinhole leaks. The engineer says there is an electrical problem that is causing the pipes to lose metal. Do you run a chain through the wheel that contacts the floor......giving some stray electrons a chance to pass through the bearings to the rim to the floor? It's a reach....a really long one at that. But, wierder stuff has happened!

[smiller]

Do you run a chain through the wheel that contacts the floor......giving some stray electrons a chance to pass through the bearings to the rim to the floor?

Why yes, I do. That must be it.

[matanuska]

...especially 'dissimilar' metals...

In this case then, I would more expect to see some wierd corrosion between say the steel outer bearing race and the aluminum bearing pocket, not between the steel balls and steel races. Of course, without a complete metalurgical analysis, there's now way to know for sure. As you point out, wierder things have happened!

 

DR

[KMG_365]

Do you run a chain through the wheel that contacts the floor......giving some stray electrons a chance to pass through the bearings to the rim to the floor?

Why yes, I do. That must be it.

Okay, this is just too weird!! How do you install a "sacrificial anode" on your motorcycle??

 

We have a similar issue in our water tanks on our Fire Apparatus (you actually have the same thing in your water heater at home as well). The different metals in the pump and the tank, which are connected electrically by the water that fills both parts (which is also filled with charged ions) which is connected to the vehicles ground (the frame) will cause an electrolysis reaction that slowly "consumes" the softest metal that will most readily give up its atoms to combine with the charged ions in solution. It is solved by adding cathodic protection to the system: sacrificial anodes (metal rods suspended in the water from the top of the tank, usually magnesium or aluminum) the softer metal in the anodes are sacrificed to the ravages of eletrolysis sparing the expensive bits. They are also much more cheaply and easily replaced than taking the Fire Apparatus out of service to disassemble the pump and rebuild/replace all the inner parts.

 

Boat owners (as well as other science geeks like me) should check out this link for more great discussions on related phenomena--INTERESTING!

[John Dickens]

I don't think it's corrosion. It looks to me like flaking of the surface treatment or case hardening. It's caused by high loading or lack of lubrication or eventually just plain wear and tear.

[smiller]

Why yes, I do. That must be it.

For those unfamiliar with my wry sense of humor I was kidding... although research reveals that electrical pitting is a possible failure mode (but it usually occurs in bearings used in some sore of electrical device.)

 

OK, I think I have my answer and John was zeroing in on it. From info gained from this interesting site it would appear the failure was (not surprisingly) caused by 'normal fatigue failure' (which is a less severe version of 'excessive load' which is why it originally seemed to me that the problem was related to being overloaded.)

 

One thing I learned from some research is that the end of service life of a roller bearing (that is properly selected and sized for the application) is not a result of simple wear and development of excessive running clearance but rather by actual metal fatigue from age... just as what happened here. From this I would draw the conclusion that checking for radial or axial play in roller bearings is not a very useful way of determining their serviceability, and that simple age is an important factor.

[Halk]

I can vouch for age taking its toll. If I knew I was going to live this long, I would have taken better care of myself. And I'm not that old!

[SWB]

What the picture demonstrates is common spalling as a result of material fatigue. Likely, at the mileage you quoted, it's normal wear and tear.

 

It could be caused by overloading AND wear. Lots of 2-up riding with gear?

 

See "Bearing Failures & Their Causes"

[smiller]

Lots of 2-up riding with gear?

Not really. Perhaps high speeds could contribute?

 

I'm thinking I pretty much just got to the end of life for the bearing. I note the following from the (excellent) link you provided:

 

"Flaking is a relatively long, drawn-out process and makes its presence known by increasing noise and vibration levels in the bearing. Therefore, as a rule, there is plenty of time to prepare for a change of bearing."

 

That was exactly my experience in this case. I noticed a barely perceptable roughness at the last tire change and then about 12k miles later it was much more noticeable, so much so that I wanted to replace the bearing. But the degredation did seem to be gradual.

 

This all seems to have implications for avoiding a 'sudden' crown bearing failure. I'm going to very carefully check it for roughness at every rear tire service from now on. Hopefully this might enable me to detect an impending problem well in advance of a total failure.

 

[Jim VonBaden]

Lots of 2-up riding with gear?

Not really. Perhaps high speeds could contribute?

 

I'm thinking I pretty much just got to the end of life for the bearing. I note the following from the (excellent) link you provided:

 

"Flaking is a relatively long, drawn-out process and makes its presence known by increasing noise and vibration levels in the bearing. Therefore, as a rule, there is plenty of time to prepare for a change of bearing."

 

That was exactly my experience in this case. I noticed a barely perceptable roughness at the last tire change and then about 12k miles later it was much more noticeable, so much so that I wanted to replace the bearing. But the degredation did seem to be gradual.

 

This all seems to have implications for avoiding a 'sudden' crown bearing failure. I'm going to very carefully check it for roughness at every rear tire service from now on. Hopefully this might enable me to detect an impending problem well in advance of a total failure.

 

Basically that is what happened to me.

 

 

Mine only had 8K miles on it, and it was a replacement of one with 40K miles on it.

 

You never know when they will go, but at least I had enough warning that I wasn't stranded.

 

Jim

[RFW]

This has nothing at all to do with "electrical" problems or any other related, weird phenomena that have been mentioned.

 

This problem is called "brinelling" or "spalling". It is the most common type of failure exhibited by ball bearings (and related high load devices such as constant velocity U-joints).

 

Ball bearings have extremely high point-loading (all the load at each ball is concentrated at one single point, resulting in astronomially high compression loading).

 

The race slightly "crushes" under each ball as it rolls by. This compression is still below the yield point of the metal, but after millions and millions of repetitions, local fatique of the race's metal (and often of the ball's metal) sets in. Then the ball starts "ripping" out bits of crushed, fatigued metal from the race as it rolls past, resulting in what you see.

 

This type of failure is most common on slow rotating, highly loaded bearings such as wheel bearings. Excessive loading adds to the problem. Insufficient lubrication is not generally a direct cause, but can add to the problem.